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- 28 - even separate the cloud top from the lower cloud part, thus disconnecting it from the water flux - and no rain enhancement can be expected. The same negative outcome for hail suppres- sion may arise because the potential for hail formation can be enhanced by incr~ased up- drafts if the embryo number is not carefully controlled such that competition can come into play. Thus, the timing location and the amount of seeding material dispersed irt a cloud are extremely important, but not enough is known about this aspect of modification optimization. ~ In a cloud which already reaches levels of low temperatures, the rele~se of latent heat may not be that important. However, cloud tops could be raised through the thermal feedback and make the cloud conditions more favourable for precipitation. This type of concept is called dynamic seeding. Dynamic-thermodynamic consequences of seeding have been considered for hurricane moderation where the formation of a second eyewall (consisting of very active convective clouds) further out from the existing one could reduce the maximum wind and thus a major factor for hurricane damage. Prevention of warm fog by heating with burners like at Orly Airport (Sauvalle, 1976) is another "brute force" method to burn up or evaporat~ the cloud droplets by application of large amounts of energy. This is still cheaper than: to disperse large CCN's (like NaC~) and have the fog droplets fallout as acid rain which w~uld do a lot of damage to aircraft by corrosion. Stirring up of cold air covering orchards - mostly as a direct or indirect result of radiation cooling - has proven to be very satisfactory through the use of large fans. At other places heating or continuous icing (release of latent heat of fusion) pave proven to be very efficient. The area of frost prevention, however, is not normally considered part of weather modification. 8. Conclusions The expansion of our knowledge in cloud dynamics and cloud microphysifs during the past three decades has been enormous. The advances, however, are not yet such that all the details of precipitation formation including intricate feedbacks are understood. This limitation also has its consequences in terms of weather modification. A few principal concepts are recognized (may be there are others?) but we still have a long way' to go to apply them properly and timely in order to get the most efficient results. ! One other problem should be mentioned: the proof that a weather modification activity has worked. Due to the state of the art of cloud physics and weather fuodification any proof needs to be statistically sound and physically satisfactory. The startdards of acceptability have been considerably increased in the past years and they are harder to satisfy. Thus, one also has to consider the possibility that weather modification activi- ties might have been successful, but the criteria applied for the judgement may; not have given a definitive answer. , 1 I Acknowledgements. The author is grateful for the support by the Atmospheric Environment Service and the National Research Council, both of Canada, and the U.S. National Oceanic and Atmospheric Administration, Department of Commerce through the National Severe Storms Laboratory, Norman, Oklahoma.